Pressure measuring apparatus



Oct. 12, 1948.

Filed July 2'7, 1944 .E. DAWSON ETAL PRESSURE MEASURING'APPARATUS 3Sheets-Sheet 1 INVENTORS I ED W400 oAWsozV u N R. ERIcsO/V ATTORN EY E.DAWSON ET AL PRESSURE MEASURING APPARATUS 3 Sheets-Sheet 2 Filed July27, 1944 mm Q v Sn WM. fix No E R m W Y IW mm .Q\ R\ m mi R .b.\ v :v rt 1 ,2: g E w WWW. m mw u L|.Q\ .w m J kw mm li 9w W. m w k\ \b v 5 m\\llll 1 fix Q 9% V 3 MHH a A E; :21 mm a w my 6 Q F1 ATTORNEY Ogt. 12,1948. E. DAWSON ETAL 2,450,884

I PRESSURE MEASURIAG APPARATUS Filed. Jul 27, 1944 :s She ets-Sheet s'ATTORN'EY Patented Oct 12, 1948 PRESSURE MEASURING APPARATUS EdwardDawson, New York, and John Rowland Ericson, Westbury, N. SperryCorporation, a corporation of Delaware Application July 27, 1944, SerialNo. 546,868

. 18 Claims. 1

This invention relates generally to apparatus for measuring a variablecondition and particularly concern-s measurement of a variable pressureas well as the rate of change thereof.

It is sometimes desirable to provide relatively high torque measures ofvariable conditions as, for example, to control mechanisms accordingtovariations in the measured condition or to supply data corresponding tothe measured condition into a computer mechanism. Some applicationsrequire that the measurement be substantially continuous withoutappreciable lag, regardless of the load on the measuring instrument.This latter requirement is particularly evident when, for example, therate of change of the measured condition is to be obtained from theoutput of the measuring instrument.

, It is an important Object of this invention to provide improvedmeasuring apparatus producing a substantial torque output measure of avariable condition.

Another object of the invention is to provide improved measuringapparatus in which the output is steady and accurate, irrespective ofvariation in loads on the instrument.

A further object of the invention is to provide improved measuringapparatus for obtaining the measure of the rate of change of a variablecondition.

Another important object of the invention is to provide improvedpressure measuring apparatus having a substantial torque output measureof a variable pressure.

A further object of the invention is to provide improved pressuremeasuring-apparatus in which the output is a steady and accurate measureof the pressure, irrespective of changes in load on the apparatus. 1

A further object of the invention-is to provide improved measuringapparatus for providing an accurate measure of the rate of change of avariable pressure.

A further object of the invention is to provide improved pressuremeasuring apparatusforobtaining a measure of dynamic pressure havingsubstantial torque output.

A further object of the invention is to provide improved follow-upmechanisms for measuring apparatus whereby hunting is reduced andsmoother as well as more accurate measures are obtained.

A still further object of the invention is to provide an improvedpressure uni-t of simple construction which may be economicallymanufactured and easily assembled.

Y., assignors to The Other objects and advantages will become apparentfrom the following description taken in connection with the accompanyingdrawings, wherein Fig. 1 is a schematic diagram of mechanisms andcircuits arranged for measuring static presthe line 4-4 of Fig. 2;

Fig. 5 is an enlarged sectional view tact members shown in Fig. 2;

Fig. 6 is a sectional view of a modified support and connection for thepressure responsive bellows shown in Fig. 1 which is adapted to measurestatic pressure; and

Fig. 7 is a schematic diagram of mechanisms and circuits embodying theinvention in a modified form for obtaining measures of static pressureand the rate of change thereof.

According to the invention, a sensitive device responsive to thecondition being measured applies a force to a lever concomitant with themagnitude of the measured condition. A spring applies an opposing forceto the lever so it moves of the conabout a fulcrum in one direction orthe other to operate contact members that are arranged to control afollow-up circuit for adjusting the tension of the spring until theforces applied to the lever are in equilibrium.

A particular follow-up arrangement found to operate satisfactorilyincludes a constant speed motor and an intermittently operable motordriving two inputs of a difierential. The speed of the intermittentlyoperable motor is controlled by a contact on the lever to increase ordecrease, dependent upon whether the force applied to the lever by thespring is greater or less than the force applied by the sensitivedevice. The output oi the differential which is driven according to'thedif- 'ference between the speeds of the two motors adiusts the tensionon the spring in a manner to restore an equilibrium condition betweenthe two forces applied to the lever.

To provide cyclic operation of the follow-upv mechanism, a disturbingforce is periodically applied to the lever to cause oscillationsthereof.

These oscillations make and break the contact periodically, so the'speedof the intermittently operable motor is controlled according to the timeduring which the contact is closed. A change in either of the forcesapplied to the lever changes the normal position of the lever andthereby changes the tlme'in'terval during which the contact is closed,so the speed of the intermittently operable motor varies to adjust thespring toward an equilibrium condition.

To measure static pressure, an evacuated bellows is connected to one endof the lever arm and is included together with the spring and the leverin an air-tight casing which may be connected as by an appropriateconduit to the static pressure being measured. In measuring dynamicpressure, the interior of the bellows is connected to the staticpressure and the casing tothe total pressure, so the force applied bythe bellows to the lever depends upon the difference between the totalpressure and the static pressure which difference corresponds to thedynamic pressure. Hence, the force applied by the spring to equalizethat applied by the bellows is a measure of the dynamic pressure.

The rate of change of pressure is obtained by connecting the output ofthe differential in the follow-up system to a suitable differentiating.mechanism which may include apparatus for operating a variable speeddevice, so its output has a rate corresponding to the output of thedifferential. When this occurs,the position of the rate control of thevariable speed device provides a measure of the rate of change of .thepressure condition.

When it is desired to obtain the rate of change of pressure as well asof the pressure itself, a modified form of the invention may be usedwhere the follow-up system includes a variable speed device having itsrate controlled according to the differential between the rates of theconstant speed motor and the intermittently operable motor; the outputof the variable speed device adjusting the tension of the spring. Withthis arrangement, the adjustable member of the variable .speed deviceprovides a measure of the rate of change of the variable pressure,whereas the output of the variable speed device, which corresponds tothetension of the spring, provides a measure of the pressure conditionitself.

Referring now to Fig. 1, there is shown apparatus embodying theinvention which provides relatively high torque output measures ofdynamic pressure, static pressure and the rate of change of staticpressure. In this figure, static pressure from a conventional Pitot tubeH is supplied as by conduit I2 to the interior of a bellows ll of adynamic pressure unit H. The bellows I3 is secured to frame ii of thepressure unit H and is connected as by a flexible spring it to one armof a lever 11. The lever I1 is attached to the frame 15 .of the pressureunit by a flexible member l8, preferably of spring steel, forming afulcrum for the lever. The frame 15 has an air-tight casing and issupplied with total pressure from the Pitot tube II as by conduit 2| sothe total pressure acts on the exterior portion of the bellows II.

The arrangement of the pressure unit may be seen more clearly in Fig. 2,wherein the static pressure input from the conduit I2 is shown as anipple 22 that has a threaded stem portion 23 secured to neck 25 fromthe bellows i! as by setscrew 26 in collar 21 to form a support for thebellows. The stem 23 is threadedly carried in the frame I! of thepressure unit by a sleeve 28 having a shoulder 20 held by brackets IIand 32 against the exterior of the frame to hold the sleeve positionedin a hole 33 formed in the frame. With this arrangement, rotation of thesleeve 28 as by turning nut portion ll thereon moves the stem 23together with the bellows ll inwardly or outwardly with respect to theframe. The opposite end of the bellows is attached through a connectoror adapter 1: to a flexible spring I! that is secured by split fastenerI to one end of the lever II. The connector 38 is restrained fromlateral movement by a spring 42 (Figs. 3 and 4) which is secured bysimilar split fasteners ll in the connector 36 and in arm 44 on thebracket II. The arm 38 on bracket 4| serves as a stop for connector 36to prevent excessive travel of the bellows II which could deformcontacts on the lever ll. Bracket II is secured to casing II by machinescrews 4| as indicated inFigs. 2 and 3.

With this arrangement supporting the bellows l3. spring 42 permitslongitudinal movement of the bellows but prevents lateral movementthereof, whereas spring I. provides a rigid connection that issubstantially free from play but sufficiently flexible to permitrelative movement between the bellows and the lever during movements ofthe lever on its fulcrum ll. These flexible spring supports andconnectors reduce friction, thereby avoiding static frictional loads onthe apparatus.

As shown in Fig. 3, the frame I! has its sides enclosed by cover plates45 and 48 to form a substantially air-tight container that is connectedas by nipple 41 to total pressure conduit 2|. Since the interior of thebellows is supplied with static pressure from the conduit I2 and theexterior thereof with total pressure from conduit 2|, the net forceexerted by the bellows through flexible spring 18 on the lever I!corresponds to the dynamic pressureat the Pitot tube I i. A coiledtension spring 40 is connected to the arm of the lever I! as by hook iion rod 51 to apply a force to the lever, counterbalancing that appliedby the bellows 'i 3. When these forces are equal, the tension of thespring It is a measure of the dynamic pressure.

To adjust the tension of the spring 48, a rod 54 is slldable in astufllng box I! carriedby the frame IS. The rod 54 has one end,extending externally of the frame l5, formed with a rack 5! adapted toengage a pinion 51 which is rotated automatically in a mannersubsequently to be described. The interior end of the rod 54 is formedwith a head 58 to retain a steel sleeve 5! on the rod 54. The sleeve 59has an enlarged threaded end portion I arranged .to engage coils at theend of the spring I! thereby connecting the spring 48 through the sleeve5! to the rod 54, so the tension of the spring I! is controlled by theposition of the rack 88 as determined by pinion 51. Since thedisplacement of the rod I4 and the rack "determines the tension of thespring M, this displacement, when forces applied to the lever H are inequilibrium, provides a measure of the dynamic pressure.

In order that this measure of the dynamic pressure may be as accurate aspossible, it is necessary that the forces applied to the lever I! shouldbe unaffected by variations in other conditions than the pressureapplied to the bellows II or the tension of the spring II. To compensatefor expansion of the beryllium copper bellows it due to temperaturechanges, .the rod N is preferably selected of material having nearly thesame coefficient of expansion. The length from the average point ofcontact between rack It and pinion I! to the In order to obtain thedesired calibration factor,

the effective length of the spring may be adjusted to the correct valueby relatively turning the sleeve 58 in the spring 48. To adjust themechanism for zero reference, the force applied by the bellows for agiven pressure may be varied by turning the nut 34 to movethe. bellowsrelative to the frame as previously explained.

When the force applied by the'bellows is more than that applied by thespring, contact members 88 and 84 engage to control the motor as will besubsequently explained. In order that contacts 88 and 84 may remainclosed during small movements of the lever I1 and the full pressure fromthe bellows I3 will not be applied to the contacts, one of the contacts63 is spring-loaded as shown in Fig. 5. For this, purpose, the contact88 is slideable in a hole 85 drilled in the lever I1 and is forcedoutwarly by a compression spring 88 engaging a button 81 maintained inposition by washer 88. It is essential that the contact 64 be insulatedfrom the casing I5 and: is desirable that changes in temperature do notaffect the spacing between the contacts 88 and 84.

For this purpose, the contact 84 is supported on the end of a conductingrod 88 extending through the casing I5 and having a suitable terminalconnection 1I. The rod 8-8 carries a fiber washer 12 engaging ends of asteel bushing 13 inserted in the casing I 5. The contact '84 is heldagainst the washer 12 by a compression spring 14 seated in a recess inthe bushing 13 and engaging an insulating fiber washer 15 on theopposite end of the rod. With this arrangement, changes in the length ofthe bushing 13 and the rod, due to changes in temperature, cause the rodto move in the bushing 12 and thus maintains substantially constant thespacing between contacts 88 and '84.

Contact 83 is grounded through the lever I1 in the casing, whereascontact 84 is connected through resistor 8I to grid 82 of an electrondischarge tube 83 having its plate 84. connected through a coil 85 ofrelay 88 to a suitable source of positive potential, such as battery 81,the negaspeed motor 88 through shaft 88, gearing III and shaft I02. Theoutput of the differential which is formed by shaft I08 operates throughgearing I04 to turn shaft I05 which controls pinion 51 to move rack 58for changing the tension of the spring 48. With this arrangement,tension of the spring 48 is adjusted according to the difference betweenthe speeds of the motors '85 and 88 and the angular position of shaftI05 is transmitted through gearing I08 to shaft I01 whose angularposition, in turn, represents a measure of the dynamic pressure. 7

In order to provide stable operation of the follow-up apparatusincluding motors 85 and 88 which are controlled by the contacts 83 and84 to quickly and smoothly position the shaft I05 for maintaining anequilibrium, condition between the forces of spring 48 and bellows I0, aharmonic disturbing force is applied to the lever I1, thereby causingoscillation of the lever and periodically opening and closing contacts83 and 84. This periodic disturbing force is shown as being applied by aspring I08 that is connected to the lever I1 by a stub I08 (Fig. 5),which is secured to the lever I1 by the washer 88. The other end of thespring I08 is connected to a rod III mounted for reciprocation in asleeve formed in support 2. The rod III is reciprocated by turning'shaftII3 -from constant speed motor 88 (Fig. 1). The shaft II3 rotates aneccentric II4 which, in turn, reciprocates link H5 connected to the rodIII for continuously applying harmonic motion for changing the tensionof the spring I08, thereby periodically applying a harmonic disturbingforce to the lever I1.

This harmonic disturbing force causes oscillation of the lever I1 so thecontacts 63 and 84 are continuously opened and closed, the period duringwhich they are closed being determined by the relative forces applied tothe lever I1 by the bellows I8 and the spring 48. As already described,the time interval during which the contacts are closed determines thespeed of the motor '85. When the force from the bellows I3 is greaterthan that from the spring 48 on the lever I1, the contacts areclosed'for a greater tive side of the battery being grounded. Cathode I88 of the tube 83 is connected to a cathode resistor 88 to ground and anegative bias is normally applied to the grid 82 by the drop across.

resistor -8I that is connected through resistors M and M. When thecontacts 88 and 84 are closed, the negative bias is removed from grid 82by grounding resistor 8|. The tube 88 then conducts current to operaterelay 88 in a manner to close relay switch 83 which closes a powercircuit from battery 84 through intermittently operable motor 85. Thespeed of the motor for. a reason which will appear as the descriptionproceeds is controlled by the time during which the relay switch 88 isclosed which, in turn, depends upon the time during which contacts 83and 84 are closed.

The motor 85 drives through shaft 88 forming one input of a subtractingdifferential 81, the other input of which is driven from constant periodof time causing the motor to exceed the speed of the constant speedmotor '88, so the output of the differential 81 turns the shaft I05 tomove rack '58 outwardly with respect to the cas ing of the pressureunit, thereby increasing the tension of the spring 48 until anequilibrium is reached.

At this time, the time interval during which the contacts are closedwill be 'just suflicient to cause motor 85 to rotate at the same speedas the constant speed motor 88. Shafts I05 and I01 rapidly andaccurately provide a measure of the dynamic pressure with a substantialtorque output from the shaft I01. The dither apparatus improves thepressure measuring operation, in that it reduces hunting of thefollow-up system so the angular position of the shaft I01 remainssubstantially constant for a constant pressure condition.

In order to prevent excessive movement of the rack 58, limit switchesI2I and I22 are placed in the circuits of the intermittently operablemotor 85 and constant speed motor 88, respectiv'ely. These switches areoperated by cams I28 and I24 arranged to open one switch or the otherwhen a desired amount of displacement of the rack in one direction orthe other is reached.

When the circuit of one of these motors is open,

to the lever I42.

the output of the differential 411s equal to the .output the motor thatis running, so the rack pressure from the Pitot tube I I is supplied byway of conduit I3I to the interior of air-tight casing I32 of staticpressure unit I33. Bellows I34 supported within casing I32 is evacuatedand hm its interior sealed off, as shown in Fig. 6, by a. cap I35 onthreaded stem I38 carried by the bellows I34. The cap I85 also serves asa support for the bellows I34, since it is fitted in a hole I31 formedin the casing I32 and is held in position by a shoulder I38 on the capI35.

The static pressure unit I33 is otherwise substantlally the same as thedynamic pressure unit I4 previously described. The bellows I34 isconnected by a flexible spring I to lever I42 secured to the casing I32by a flexible spring I43 forming a fulcrum for the lever. Tocounterbalance the force of the bellows I34, a spring I44 has one endattached to the lever I42 and the other end to a rod I45 slideable inthe casing I32 and carrying a rack I46 externally of the casing to meshwith a pinion I41.

A contact I48 carried by the lever I42 is adapted to engage acooperating contact I49 supported on the casing and connected throughresistor II to control grid I52 of an electron discharge tube I53.Normally, a negative bias is applied to the grid I52 by the drop acrossresistor 9i connected to grid I52 through resistors iii and I5I'. Whenthe contacts I48 and I49 are closed, the negative bias is removed fromthe grid I52 by grounding resistor I5I. This causes current to flowbetween plate I55 and cathode I56, thereby drawing current througharelay coil I51 in the circuit of plate I55 operating relay switch I58,closing the circuit of intermittently operable motor I59.

Intermittently operable motor I59 drives through shaft I6I todifferential I62 having its other input driven by shaft I42 from theconstant speed motor 98, so the output shaft I63 of the differential I62is driven according'to the difference between the speeds of the constantspeed motor 98 and the variable speed motor I59. This output shaft I63is connected to gearing I64 to shaft I65 that carries thepinion I41 toposition rack I46 for adjusting the tension of spring I44 and therebyvarying the force applied Dependent upon the relative magnitudes of theforces applied to lever I 42 by the bellows I34 and the spring I44,contacts I48 and I49 will be opened or closed to vary the speed of themotor I59. When the speed of the motor I59 equals that of the constantspeed motor 98,

position of shaft I65 is such that the spring I44 is applied with forceto lever I42 corresponding to that applied by the bellows I 34. Hence,the angular position of shaft I65 is a measure of the static pressuresupplied from conduit Hi to the casing I32. This measure of staticpressure is transmitted through gear "I to static pressure output shaftI12 where it may be utilized in any desired manner.

In order to stabilize operation of the follow-up system and to provide asmoother and more accurate measure of static pressure, a disturbingforce is periodically applied to the lever I42 as by spring I13connected to the lever I42 and having its tension constantly changed byan eccentric I14 driven from constant speed motor 44 by the shaft 8 H3.The spring I13, therefore, periodically spplies a harmonic disturbingforce to the lever I42 to periodically open and close the contacts I44and I44. The contact I43 may be spring-loaded in the same manner as thecontact 43 in the dynamic pressure unit. With this arrangement, the timeinterval during which the contacts I44 and I44 are closed durin eachperiod of the operation of spring I13 depends upon the relativemagnitudes of the forces exerted on the lever I42 by the bellows I34 andthe spring I44. Since the time interval during which the contacts I44and I44 are closed controls the speed of intermittently operable motorI44, when the two forces applied to the lever I42 are in equilibrium,the time interval during which the contacts are closed will besufficient to operate the motor I44 at the same speed as that of theconstant speed motor 34. In this condition, the output shaft I43 of thedifferential I62 is stationary and its angular position whichcorresponds to the angular position of the shaft I65 is a measure of thestatic pressure supplied to the casing I32.

In order to avoid excessivemovement of the rack I46, suitable limitswitches I15 and I14 are inserted in the circuits of constant speedmotor 98 and intermittently operable motor I46, respectively. Theseswitches are operated by cams I11 and I18 arranged on the shaft I65 toopen the switches when the rack I46 reaches its terminal position.Opening of one of the limit switches I15 or I16 stops operation ofeither the constant speed motor 98 or the intermittently operable motorI59, so the output of the differential I43 causes the shaft I65 to movethe rack I44 in the opposite direction to move it away from its limitingposition until the limit switch is closed.

It is frequently desirable to provide a measure of the rate of changefor a variable condition as well as a measure of the variable conditionitself. For example, as illustrated in Fig. 1, it may be desirable toobtain a measure of rate of change of the static pressure as measured bythe static pressure apparatus just described. To accomplish this, adifferentiating mechanism is shown as including a variable speed driveI8i having a disc I82 driven from constant speed motor 44 through shaftI82, gearing I63, shaft I84, gearing I85 and shaft I86. Motion from thedisc I32 is transmitted to a driven drum I81 by a pair ofballs I88supported in a carriage I89 displaceable radially of the disc I82 bytranslation of rack I9I. The position of the rack I9I is controlled by apinion I92 carried on a shaft I93 that is driven from reversible motorI94 in a manner to be de scribed. Since the angular position of theshaft I65 is an accurate measure of the static pressure, the shaft I65is used to position contact disc I44 carried on shaft I96 that is drivenby gearing I91 from the shaft I65. The disc I carries a pair of contactsI98 and I98 that are connected to opposite sides of a two-part fieldwinding I44 in the motor I94. The center point of the field winding I99is connected through armature 2" to a source of potential such asbattery 282 having its opposite terminal connected to ground.

A- grounded contact radial arm 263, as shown in Fig. 1A, is mounted onshaft 241, which is actuated by drag spindle or slip clutch 284 drivenby shaft 285 forming the output from the driven drum I31 of the variablespeed drive I4I, so that the position of the contact 243 tends tocorrespond change 01' the static pressure being measured, any differencebetween the angular position of these contacts and that of contact 203will cause either the contact I98 or the contact I98. to engage contact203, thereby completing the circuit from battery 292 through armature29I and onehalf of field winding I99 to ground. Dependent upon whichside of the field winding I98 is connected in the circuit, the armature20| will drive shaft I93'in one direction or the other to change theposition of the ball carriage in a manner to adjust the speed of outputshaft281 to match the speed of the shaft I96 driving contacts" I98 andI98. v

In order to preventoverrunning of the'shaf I93, a brake 209 is operatedby solenoid 2I0 to permit free rotation of the shaft when contact I98 orI98 is closed, but to stop rotation of the shaft I93 immediately whenthe contact opens.

This causes the shaft I93 tohold its position except during periods ofenergization of the motor.

When the equilibrium condition is reached where the speed of contact 293equals that of contact disc I95, displacement of ball carriage I89 willbe an accurate measure of the speed,

since it controls the speed of the drum I81. Since contact disc I95 isdriven at a speed'determlned by the change of the static pressure beingmeasured, the position of the ball carriage I89 as determined by pinionI92 from motor I94 will be a measure of the rate at which the staticpressure is changing. This measure of static pressure rate may besupplied from shaft I93 through. gearing 2 to static pressure rateoutput shaft 2I2 for use in any manner that may be desired.

Where it is desirable to obtain both measures of static pressure and therate of change thereof, a simplified arrangement, such as that shown inFig. 7, may be used. In this figure. static pressure from Pitot tube 22Iis supplied through conduit 222 to the interior of casing 223 of staticpressure unit 224 containing an evacuated bellows 225 connected as bysprin 226 to lever 221. The lever 221 is supported by a flexible fulcrum228 on the casing 223 and has a spring 229 exerting the opposing forceon the lever 221 as determined by the position of rack 23I. Dependentupon the relative magnitudes of the forces applied to the lever 221 bythe bellows 225 and the spring 229, contacts 232 and 233 areopened orclosed to cause operation of intermittently operable motor 235. Whenthese contacts are closed, the negative bias applied to grid 236 ofelectron discharge tube 231 by resistor 238' through resistors 239 and239' is removed by grounding resistor 239. The tube 231 has its plate24I connected through relay coil 242 to battery 243 so current in thecircuit of plate 24I causes operation of relay switch 248 which closesthe circuit of motor 235 through bat-.

tery 244.

The output of the intermittently operable motor, as represented by shaft245, drives into one input of a differential 246 having its other inputdriven from constant speed motor 241 through shaft 248, gearing 249 andshaft 25I. The output of the differential 246, as represented by shaft252, is thereby rotated according to the difference between the speedsof the motors 235 and 241. This shaft 252 operatesthrough pinion 253 andrack 254 to control the position of ball carriage 255 of variable speeddrive 256 having its disc 251 driven by shaft 248 from the constantspeed motor 241. With this arrangement, the speed of output drum 258 iscontrolled according to the angular displacement of. shaft 252 anddrives sure.

through shaft 259 to a difierential 26I and operates output shaft 262 ofthe differential 26I This output of the differential 26I drives pinion263 and gear 264 to position shaft 265, which operates throughdifferential 2 66 to position shaft 261 and pinion 268 cooperating.wlthrack 269 for translating rod 21I to vary the tension applied byspring 229 tov the lever 221.

From an examination of the apparatus shown in Fig. '1, it will beapparent that the motor 235 is operated by contacts 232 and 233according to the difference in magnitude of the forces applied bybellows 225 and spring 229 to the lever 221. Thes eds of the motor 235and constant speed motor 241 are subtracted in differential 246 andoperate the variable speed drive 256 to change the tension of spring229,thereby changing the force it applies to lever 221. when the forcesapplied to lever 221 are in equilibrium, the position of the rack 23Iwill be a measure of static pressure within the casing 223. Since theposition of this rack is controlled by the output of the variable speeddrive 256, the position of ball carriage 255 as determined. by pinion253 on shaft 252 will be a measure of the rate of change of the staticpres- Therefore, the shaft 252 provides a substantial torque output thatis an accurate measure of the rate of change of the static pressure fromthe Pitot tube; 22I and may be used in any desired manner.

In this embodiment of the invention, it is also desirable to apply aperiodic disturbing force to the lever 221. In this case, the disturbingforce is shown as applied through the spring 229. This is accomplishedby an eccentric 213 on shaft 214 driven by constant speed motor 241through gearmg 249. The eccentric 213 reciprocates the rod 215 carryingrack 216 engaging with pinion 211 forming a second input to thedifferential 266. Since the eccentric 213 is rotated at a constant rate,the rod 215 continuously reciprocates, thereby cscillating the pinion211 and adding this oscillation to the turning of the shaft 265 bydifferential 266 so output shaft 261 is positioned according to the sumof the output of the variable speed drive 256 and the oscillation ofthe-pinion 211. Thus, .the rack 23I controlling the tension applied tospring 229 is continuously oscillated, thereby periodically applying aharmonic disturbing force to the lever 221. The application of thisperiodic disturbing force greatly improves the operation of thefollow-up apparatus and causes contacts 232 and 233 to periodically openand close, the time interval during which they are closed controllingthe speed of the motor 235.

In order to stabilize the operation of the system, a displacement factorfrom the output of differential 246, as represented byshaft 252corresponding to the rate of change of static pressure, is supplied fromgearing 28I through shaft 282. gearing 283 and shaft 284 to.differential 26I.

Since the shaft 259 from the-drum 258 forming one input of thedifferential 26I is rotated at a speed corresponding to the rate ofchange of the pressure condition being measured and the shaft 284 isdisplaced according to the same speed, output shaft 262 of thedifferential 26I is controlled by two factors, namely, the displacementand rate of change of displacement of the shaft 252. With thisarrangement, changes in the rate at which the pressure condition isvaried provide impulses through the stabilizing connection of shaft 282to differential 26 I for rapidly positioning the shaft 261 in adirection to compensate the tension of spring 229 according to thechanges in the pres- 1 i sure without waiting for the variable speedmechanism to pick up these changes and change the speed of its outputaccordingly.

In both embodiments of the invention, a sensitive device in the form ofbellows responds to changes in the pressure condition being measured andthereby changes the force applied to a lever which acts as a controlmember connected between the bellows and a. device in the form of aspring applying an opposing force to the lever. The lever or controlmember operates contacts for controlling the speed of a. motor which issubtracted from the speed of the constant speed motor. This arrangementof controlling the speed of the motor and matching it with the speed ofthe constant speed motor has been found superior in the presentapplication to operation of a single reversing motor for the reason thatit eliminates the heating caused by high starting currents whichcontinuously appear in operating a reversible motor first in onedirection and then in the other. Smoother operation is also providedbecause the intermittently operable motor is operating on a flatterportion of its motor characteristic curve so changes in the appliedvoltage effect similar changes in speed, whereas in the starting andreversing motor changes in the applied voltage cause much largeraccelerations. Another feature incident to the use of an intermittentlyoperable motor and a constant speed motor is the requirement of only onecontrol contact to cause operation of the output in the system in eitherdirection, eliminating error due to gap width in a two-contact systemnecessary for a reversing motor.

The smoothness of operation of the pressure system is greatly improvedby the periodic application of a small disturbing force to causeoscillation of the control contacts. Since the'impulses are closetogether, the speed of the motor is instantaneously and smoothlycontrolled so the difference between the speeds of the intermittentlyoperable and constant speed motors is compara; tively smooth andprovides an even output measure of the pressure condition or the rate ofchange thereof as the case may be.

As many changes could be made in the above construction and manyapparently widely different embodiments of this invention could be madewithout departing from the scope thereof, it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

What is claimedis:

1. Measuring apparatus comprising a control member, a sensitive deviceadapted to exert a force on said control member according to thecondition being measured, an adjustable forceapplying device connectedto said control member for applying a balancing opposing force thereto,means for periodically applying a disturbing force to said controlmember, an electric motor, control circuits therefor intermittentlyoperated by said control member according to the difference between theforces applied thereto, and a variable speed device having its speedcontrolled by said motor for adjusting said force-applying device.

2. Measuring apparatus comprising a sensitive device adapted to exert aforce according to the condition being measured, a force-applying deviceconnected to said sensitive device for applying an motor intermittentlyoperated by the electric circuit means according to the differencebetween said forces, a variable speed device operated by said motor foradjusting said force-applying device, and a stabilizing connection forfurther controlling said force-applying device by the output of saidmotor.

3. Measuring apparatus comprising a control member, a sensitiv deviceadapted to exert a force on said control member according to thecondition being measured, a force-applying device connected to saidcontrol member for applying an opposing force thereto, means forperiodically applying a disturbing force to said control member, aconstant speed motor, an intermittently operable motor means actuated bysaid control member according to the difference between said forces forcontrolling the speed of the intermittently operable motor, a variablespeed device for adjusting said force-applying device, and differentialmeans driven by said motors for controlling the speed of said variablespeed device according to the diflerence between the speeds of saidmotors.

4. Measuring apparatus comprising a lever, a bellows connected to saidlever and adapted to exert a force thereon according to the conditionbeing measured, a force-applying device connected to said lever forapplying an opposing force to said bellows and means for controllingsaid force applying device including a constant speed motor, anintermittently operable motor, means for controlling the speed of saidintermittently operable motor by said lever according to the differencebetween said forces, and differential means driven by said motors foradjusting said force-applying device'according to the difopposing forcethereto,- electric circuit means,

jointly controlled by said devices, an electric ference between thespeeds of said motors to equate said opposing force to said exertedforce.

5. Measuring apparatus comprising a control member, a sensitive deviceadapted to exert a force on said control member according to thecondition being measured, a force-applying device connected to saidcontrol member for applying an opposing force thereto, means forperiodically applying a disturbing force to said control member, aconstant speed motor, an intermittently operable motor, means forcontrolling the speed of the intermittently operable motor responsive tothe differences between the forces applied to the control member, anddifferential means operated by said motors for adjusting saidforce-applying device according to the diflerence between the speeds ofsaid motors.

6. Measuring apparatus comprising a sensitive device adapted to exert aforce according to the condition being measured, a force-applying devicedirectly connected to said sensitive device for applying an opposingforce thereto, means for controlling the force-applying device includinga constant speed motor, an intermittently operable motor, meanscontrolling the speed of said intermittently operable motor according tothe difference between said forces, a variable speed mechanism foradjusting said force-applying device, and difierential means driven bysaid motors for controlling the speed of said variable speed mechanismaccording to the diiierence between the speeds of said motors. 1

7. Measuring apparatus comprising a lever, a bellows connected directlyto the lever and adapted to exert a force thereon according to thecondition being measured, a spring connected to the lever for applyingan opposing force to the bellows, means for adjusting t e force appl edy opposing force of the spring to the exerted force.

8. Measuring apparatus comprising a lever, a bellows connected directlyto the lever and adapted to exert a force thereon according to thecondition being measured, a spring connected to the lever for applyingan opposin force to the bellows, means for adjusting the force appliedby the spring to the lever including electric circuit means operable bythe lever on' displacement thereof, a motor controlled by the circuitmeans intermittently according to the difference between said forces, aconstant speed motor, and differential means driven by said motors foradjusting the spring accordin to the difference between the speedsof'the motors to equate the opposing force of the spring to the exertedforce.

9. Measuring apparatus comprising a lever, a bellows connected directlyto the lever and adapted to exert a force thereon according to thecondition being measured, a rack, a pinion in mesh therewith, a springfor applying an opposing force to the bellows having opposite endsattached respectively to the rack and the lever,

means for adjusting the forceapplied by the spring, including anintermittently operable motor, means actuated by the lever according tothe difference between said forces for controlling the motor, a constantspeed motor, and differential means driven by the motors for actuatingthe pinion to equate the opposing force of the spring to the exertedforce.

10. Measuring apparatus comprising a lever, a bellows connected to thelever adapted to exert a force thereon according to the condition beingmeasured, a rack, a pinion in mesh therewith, a spring for applying anopposing force to the bellows supported by the lever and rack, means foradjusting the tension of the spring and thereby the force applied by thespring to the lever which includes contact means intermittently operableby the lever according to the difference between said forces, circuitmeans including a motor operable by the contact means, a constant speedmotor, and differential means driven by the motors for actuating thepinion to displace the rack and adjust the spring to equate the opposingforce. of the spring to the exerted force.

11. Measuring apparatus comprising a lever, a bellows connected to thelever adapted to exert a force thereon according to the condition beingmeasured, a springconnected to the lever for applying an opposing forceto the bellows, means for acLiusting the force applied by the springincluding an intermittently operable motor, means for controlling thespeed of the motor. by the lever according to the difference betweensaid forces, a constant speed motor, differential means driven by saidmotors for adjusting the spring according to the difference between thespeeds of the motors to equate the opposin force of the spring to theexerted force, and a variable speed mechanism jointly controlled by themotors for obtaining a measure of the rate ofchange of the conditionbeing measured.

12. Measuring apparatus comprising a lever, a bellows connected directlytothe lever and adapt- .N 1.4 ed to exert a force thereon according tothe condition being measured, a spring connected to the lever forapplying an opposing force to the bellows, a dithering device directlyconnectedto the lever, means for adjusting the force applied by thespring including an intermittently operable motor, means for controllingthe speed of the motor by the lever according to the difference betweensaid forces, a constant speed motor, and differential means driven bysaid motor for adjusting the spring according to the difference betweenthe speeds of the motors to equate the opposing force of the spring tothe exerted force.

13. Measuring apparatus comprising a lever, a bellows connected directlyto the lever and adapted to exert a force thereon according to thecondition being measured, a spring connected to the lever for applyingan opposing force to the bellows, means for adjusting the force appliedby the spring including an intermittently operable motor, means forcontrolling the speed of the motor by the lever according to thedifference between said forces, a constant speed motor, dither meansactuated by the constant speed motor directly connected to the lever,

and differential means driven by said motors for adjusting the springaccording to the difference between the speeds of the motors to equatethe opposing force of the spring to the exerted force.

14. Measuring apparatus comprising a lever, a bellows connected directlyto the lever and adapted to exert a force thereon according to thecondition being measured, a spring connected to the lever for applyingan opposing force to the bellows, means for adjusting the force appliedby the spring including an intermittently operable motor, means forcontrolling the speed of the motor by the lever according to thedifference between said forces, a constant speed motor, dither meansactuated thereby directly connected to the lever, differential meansdriven by said motors for adjusting the spring according to thedifference between the speeds of the motors to equate the opposing forceof the spring to the exerted force and a variable speed mechanismjointly controlled by the motors for obtaining a measure of the rate ofchange of the condition being measured.

15. Measuring apparatus comprising a lever, a bellows connected to thelever adapted to exert a force thereon according to the condition beingmeasured, a spring connected to the lever for applying an opposing forceto the bellows, a constant speed motor, dither means actuated therebydirectly connected to the lever, means for adjusting the force appliedby the spring to the lever including electric circuit means operable bythe lever on displacement thereof, a motor operated by the circuit meansintermittently according to the difference between said forces, anddifferential means driven by said motors for adjusting the springaccording to the difference between the speeds of the motors to equatethe opposing force of the spring to the exerted force.

16. Measuring apparatus comprising a lever, a bellows connected to thelever adapted to exert a force thereon according to the condition beingmeasured, a spring connected to the lever for applying an opposing forceto the bellows, a constant speed motor, dither means actuated therebydirectly connected to the lever, means for adjusting the force appliedby the spring to. thelever including electric circuit means operable bythe auopsc -15 lever on displacement thereof, amotor operated by thecircuit means intermittently according to the difference between saidforces, diflerential means driven by said motors for adjusting thespring according to the diflerence between the speeds or the motors toequate the opposing force 01 the spring to the exerted force and avariable speed mechanism jointly controlled by the motors for obtaininga measure of the rate of change of the condition being measured.

17. Measuring apparatus comprising a lever, a bellows connected to thelever adapted to exert a force thereon according to the condition beingmeasured, a rack, a pinion in mesh therewith, a spring for applying anopposing force to the bellows supported by the lever and rack, aconstant speed motor, dither means actuated thereby directly connectedto the lever, means for adjusting the tension of the spring and therebythe force applied by the spring tothe lever which includes contact meansvariably and intermittently operated by the lever according to thedifference between saidiorces, circuit means including a motor operatedby the contact means, and diflerential means driven by the motors foractuating the pinion to displace the rack to equate the opposing forceof the spring to the exerted force.

18. Measuring apparatus comprising a lever, a bellows connected to thelever adapted to exert a force thereon according to the condition beingmeasured, a rack, a pinion in mesh therewith, a spring for applying anopposing force to the bellows supported by the lever and rack, aconstant speed motor, dither means actuated thereby directly connectedto the lever, means for adjusting the tension of the spring and therebythe force applied by the spring to the lever which includes contactmeans variably and intermittently operated by the lever according to thedifference between said forces, circuit means including a motor operatedby the contact means, diflerential means displace the rack to equate theopposing torce oithe spring to the exerted force, and a variable speedmechanism jointly operated by the motors for obtaining a measure 0! therate of change of the condition being measured.

EDWARD DAWSON. JOHN ROWLAND ERICSON.

REFERENCES CITED 7 The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,370,193 Crocker Mar. 1, 19211,487,402 Roucka Mar. 18, 1924 1,512,805 Roucka Oct. 21, 1924 1,585,170Roucka May 18, 1926 1,633,713 Roucka June 28, 1927 1,638,100 Roucka Aug.9, 1927 1,638,101 Roucka Aug. 9, 1927 1,641,196 Roucka Sept. 6, 19271,641,198 Roucka Sept. 6, 1927 1,681,309 Paulin Aug. 21, 1928 2,098,574Doyle Nov. 9, 1937 2,107,976 Blasig Feb. 8, 1938 2,115,351 Terry et alA1) 28, 1938 2,151,941 Rydberg Mar. 28, 1939 2,185,578 Beardsley et alJan. 2, 1940 2,217,638 Luhrs Oct. 8, 1940 2,269,068 Corbin Jan. 6, 19422,337,152 Clewell Dec. 21, 1943 2,352,312 Donaldson June 27, 19442,383,758 Ziebolz Aug. 28, 1945' FOREIGN PATENTS Number Country Date248,384 Great Britain May 25, 1927 681,544 Germany Sept. 25, 1939708,653 France May 4, 1931

